1. Field of the Invention
The present invention relates to a photo detector, an image sensor, a photo-detection method, and an imaging method.
2. Description of the Related Art
Single-crystal silicon (Si) is mainly used as the photoelectric conversion material of photo detectors in the visible range. Silicon (Si) has an internal photoelectric effect with light having a wavelength of 1,100 nm or less, and produces electron and hole pairs inside the crystal. Such an electron and hole pair can be separated by a potential barrier due to a junction or a bias voltage, and extracted, thereby allowing incident light to be detected as an electric signal. Silicon (Si) has a photo sensitivity over a wide range, from X rays to ultraviolet rays to near-infrared rays. However, for some uses of photo detectors, it is desired that sensitivity is only provided for a certain wavelength. To achieve this, a color filter is provided in front of a photodiode used as a photoelectric conversion device.
In a two-dimensional image sensor, color separation is generally performed using color filters of primary colors or complementary colors arranged in the form of a mosaic. However, from the standpoint of a need for increasing the number of pixels (i.e., increasing the number of pixels or decreasing the size of pixels) to achieve a high resolution for the sensor, the size of the sensor structure has not been changed in the height or depth direction, but the size in the in-plane direction has been reduced. Consequently, the structure of each pixel of the sensor has a high aspect ratio (i.e., the ratio of the height or depth of the sensor to the diameter of a light-receiving surface is large). Accordingly, eclipsing and color mixing easily occur. Furthermore, the size of such color filters has also been reduced with decreasing pixel size. Accordingly, forming different color filters for small areas makes it difficult to decrease the cost.
Consequently, in order to prevent color mixing, Japanese Patent Laid-Open No. 2003-142674 discloses a MOS-type solid-state image sensor in which photoelectric conversion regions and signal scanning circuit regions are separated by trench isolation regions, wherein the sensor includes element isolation diffusion layers provided at a position deeper than a photodiode diffusion layer below the bottom surface of the trench isolation regions. This arrangement blocks leakage current between pixels and suppresses color mixing. Japanese Patent Laid-Open No. 2007-13065 discloses a near-infrared photo detector in which photonic crystals, which are designed so as to have a periodic-refractive-index structure having a resonance peak in a near-infrared wavelength range, are provided on a photoelectric conversion region. This photo detector realizes a high-speed photo detection operation and an improvement in the quantum efficiency.
In order to obtain a color image, color information is necessary for each of the very small blocks (pixels) used for forming the image. All colors are formed by a mixture of three colors of red (R), green (G), and blue (B), and differences in the colors are caused by differences in the mixing ratio of the three colors. That is, in order to obtain a color image, the light intensity ratio of the three colors of RGB is detected for each block. In a still camera, color filters that transmit only one of the three colors of RGB are disposed in the form of a mosaic in front of a detector, and an interpolation operation is performed on adjacent pixels on the basis of the intensity ratio. Accordingly, the intensity ratio of the three colors of RGB for each pixel is determined, and thus, a color image is obtained. This method is widely used. Regarding the arrangement of color filters, color filters having the Bayer arrangement, in which a certain row has an arrangement of RGRG and the next row has an arrangement of GBGB, or color filters having a stripe of G and a checkered arrangement of RB are often used.
However, image sensors utilizing an arrangement of color filters has some problems. One of the problems is the loss of light caused by using color filters. For example, light that can pass through a red color filter is composed of a spectrum component corresponding to red light. Other spectrum components corresponding to blue and green light are absorbed by the filter, and, thus, these spectrum components are not utilized in the formation of an image. When the resolution of an image sensor is further increased in the future to decrease the size of pixels, the S/N ratio will be a matter for concern and the loss of light will not be negligible. Another problem is the decrease in the resolution caused by a low-pass filter. When an image composed of different colors contained in very small areas, e.g., substantially spaced at intervals of pixels, is input to an image sensor utilizing a color filter arrangement, false colors, which are different from the original colors of the image, are generated. In order to suppress generation of these false colors, it is necessary to gradate the image using an optical low-pass filter.
The image sensor disclosed in U.S. Pat. No. 5,965,875 is an example of an image sensor that compensates for the problems associated with the use of color filters. The specification of this patent document proposes a photo-detection method that uses a two-dimensionally laminated image sensor in which a pixel has a multilayer structure and the three colors of RGB are detected at different depths utilizing differences in the absorption coefficients of Si. In this two-dimensionally laminated image sensor, a high S/N ratio can be expected because photo detectors having a spectroscopic function are arranged and loss of light due to the presence of a color filter does not occur. Japanese Patent Laid-Open No. 2005-268609 discloses a method of laminating organic photoelectric conversion layers having different absorption wavelength ranges. This method senses an image composed of three colors of RGB using such a laminated structure, and an optical low-pass filter is necessary to suppress an aliasing distortion at the Nyquist frequency or lower frequencies. However, in this method using the laminated structure, the cutoff frequency can be higher than that in the method using color filters, and absence of high-frequency components can be suppressed.
In the image sensor described in Japanese Patent Laid-Open No. 2003-142674, since a photoelectric conversion layer (photodiode) is formed so as to extend to a shallow position, the sensitivity is sacrificed. As a result, the image sensor may not be sensitive particularly to red (R) light for which the absorption coefficient of Si is low. In the photo detector disclosed in Japanese Patent Laid-Open No. 2007-13065, since the periodic structure is composed of SiO2 having a low loss, a resonance peak becomes steep, and this structure is not necessarily suitable for color separation of visible light.
In the device disclosed in U.S. Pat. No. 5,965,875, a two-dimensionally laminated image sensor includes arranged photo detectors having a spectroscopic function, and performs spectral separation utilizing differences in the absorption coefficients of Si for different components of light. Therefore, this device is disadvantageous in that the film thickness of the device is large, and color mixing easily occurs when light is incident at an angle (see a description below of the case where a thick photo detector 902 is used, as shown in FIG. 9B). Furthermore, this device is disadvantageous in that it is difficult to achieve satisfactory color separability together with a high sensitivity.
In the laminate image sensor disclosed in Japanese Patent Laid-Open No. 2005-268609, color separability can be improved by spectral separation performed by adjusting the spectral sensitivity of an organic dye. However, this sensor has a problem of chemical instability, because an organic substance is used. In addition, this sensor is disadvantageous in that the sensitivity is not satisfactory because of its low carrier mobility. Furthermore, this sensor also has a problem in terms of the incompatibility of a production process of the sensor with a production process of peripheral circuits, such as a readout circuit and an amplifying circuit.